1. Field of the Invention
The present invention relates generally to systems and methods for the recovery of subterranean deposits, and, more particularly, relates to systems and methods for the removal of inseam water, drilling effluent and the production of gas, typically methane, from a gas bearing strata.
2. Background of the Invention
Coal is a large energy source. It has been mined from the earth for many years. Deposits of coal beneath the ground surface are positioned in generally horizontal coal seams and include substantial quantities of methane gas entrained in the coal deposits. In underground coal mining, methane gas poses a significant safety risk to the miners. In the past, the methane gas entrained in the coal deposits was simply liberated from the coal, mixed with air in the mine which diluted it to a safe concentration, and the mixture was ventilated to the outside environment. The methane was simply dissipated into the environment and provided no meaningful resource. However, in recent years, this entrained methane gas has been an efficient energy source and is sold commercially. Typically the methane gas is used as a driving source for energy-producing equipment, such as generators or the like, or can be added to natural gas pipelines.
Utilizing the gas as an energy source requires that the gas be extracted in a concentrated state and captured. Extracting methane from the coal seams in a concentrated state has been achieved by drilling boreholes generally horizontally into the coal seam that can extend several thousands of feet.
During and after the methane drilling process, dewatering must occur. Since coal seams may have a significant amount of subterranean water associated with them, water must be drained from the coal seam in order to produce the methane. Further, during the drilling process, water may be used at the drilling tip, creating a slurry of drill cuttings, which also must be removed from the borehole. Water and drill cuttings can block the migration of gas through the coal seam to the borehole and therefore must be removed to permit degasification. Additionally, some of the water used in the drilling process can be forced under pressure into the coal seam, further saturating the gas reservoir, which impedes the migration of gas to the borehole. Therefore, dewatering must occur both during the drilling process and after drilling has been completed.
Long, generally horizontal boreholes that remain in the coal seam are the most effective manner to extract and capture the gas entrained in the coal seam providing the suitable reservoir and material strength characteristics exist in the coal seam. Horizontal and generally horizontal boreholes can be effective in a suitable coal seam because they remain in contact with the gas reservoir (the coal bed) for long distances. Typical generally horizontal directional boreholes are drilled from inside the coalmine, which are relatively easy to dewater. This is due to the ability of the gas to purge water from the borehole because the boreholes are generally level with the end of the borehole, therefore, the gas does not have to overcome substantial hydraulic head to purge the water from the borehole. However, the horizontal directional boreholes drilled from inside the coal mine create several safety concerns, require the use of specialized equipment, and usually have limited borehole productive life. Transportation of gas in a pipeline, inside an underground coalmine, requires considerable maintenance and safety inspections due to the explosive nature of gas. Although directionally drilled inseam boreholes can reach several thousand feet, the boreholes do not always provide complete degasification before the coal seam is mined. Oftentimes the coal mining operator must mine the coal in an area before maximum degasification has been achieved. Therefore, a method to maximize coal bed gas recovery while reducing the safety risks to the coal operators is desirable. Furthermore, in areas not associated with current or future coal mining, the dewatering methods described herein, both during and after drilling will maximize coal seam methane recovery.
It is an object of the present invention to overcome the deficiencies inherent in the prior art. It is another object of the present invention to provide a method for producing gas from a coal seam yielding increased operator safety. It is another object of the present invention to provide a method for producing gas from a coal seam while simultaneously producing in-situ water, water induced in the coal seam, and drill cuttings while drilling by the drill mechanism during the directional drilling operation. It is yet another object of the present invention to provide a method of simultaneously producing gas and water from a coal seam after drilling is complete to allow for increased gas exploitation of a coalfield.